Ferrierite is a natural zeolite of the approximate formula: (Na,Mg).sub.x/3 (AlO.sub.2).sub.x (SiO.sub.2).sub.36-x 8H.sub.2 O characterized by a value of x close to 6. The structure of a crystallized zeolite is formed of a frame resulting from the linking of TO.sub.4/2 tetrahedrons, each oxygen being common to two tetrahedrons. T is mostly silicon or aluminum, but other trivalent elements such as boron, gallium or iron may be introduced instead of aluminum into TO.sub.4 tetrahedons of the frame of certain zeolites. The presence of these trivalent elements, together with tetravalent silicon, gives negative charges to the frame, said charges being neutralized by compensation cations. The microporous volume left free by the frame is occupied by exchangeable compensation cations and different chemical species such as water, amines, alcohols, salts, bases etc. It is important to observe that all these species, by their introduction during the zeolite formation, are responsible for its microporous structure. As a matter of fact, by their shape, their size and their interactions, they make it possible to build light frames comprising voids of variable volume and shape.
The use of zeolites as microporous solids for selective adsorption or for catalysis requires at least a partial removal by heating or roasting of the species introduced during the synthesis. Each type of zeolite has then a distinct microporous structure related with the geometry of the frame and with the number and the size of the present compensation cations. The variation in size and shape of the channels and recesses from one type to the other results in changes of the adsorbing properties. Only the molecules having certain sizes and shapes are capable to penetrate the pores of a given zeolite. In view of these remarkable characteristics, zeolites are particularly convenient for the purification or separation of gas or liquid mixtures such for example as hydrocarbons separation by selective adsorption.
The chemical composition with the nature of the elements present in the TO.sub.4/2 tetrahedrons and the nature of the compensation cations is also an important factor having an effect on the selectivity of adsorption and mainly on the catalytic activity of these products. This is explained by the nature and the intensity of the interactions between zeolites and the molecules adsorbed on their internal surface. They are used as catalyst carriers or catalysts for cracking and modifying hydrocarbons as well as in the preparation of many molecules.
Among these different types of zeolites for industrial use in separation or catalytic processes, ferrierite is very using. With a relatively high Si/Al ratio, its structure has a high stability. For the same reasons, it has a strong acidity when the compensation cations are H.sup.+ protons and its stability remains acceptable. Finally, the geometry of its microporous system and the size of the micropores (4.3.times.5.5 A in the direction (001) and 3.4.times.4.8 A in the direction (010) with interconnection of the two channel systems) provides a microporous solid having a good selectivity in adsorption and catalysis operations.
Fields of natural ferrierites are known, but the industrial use of these products is limited by the variable quality of the products and the presence of impurities forming undesirable charges in certain processes. On the other hand, some applications require higher Si/Al ratios than that of natural ferrierites, which range from 4 to 7. As a rule, it is of course possible to increase this ratio by different chemical treatments but these processes are time-consuming and costly. On the other hand, dealumination of ferrierite by post synthesis treatments known in the prior art has proved to be very difficult, or even impossible (G. FERRE, Doctor-Engineer Thesis, ENSPM 1986).
These problems have initiated substantial research in an effort synthesize ferrierites having the desired qualities and properties. These syntheses are disclosed in many patents and papers, such for example as paper of R. M. BARRER and D. J. MARSHALL (J. Chem. Soc. 1964 p. 485) and the following patents: FR No. 2 228 721 (1974); U.S. Pat. No. 3,966,883 (1976); Ger offen No. 2 548 697 (1976); U.S. Pat. No. 3,933,974 (1976); U.S. Pat. No. 4,016,245 (1977); U.S. Pat. No. 4,017,590 (1977); U.S. Pat. No. 4,088,739 (1978); EP No. 12 473 (1980); U.S. Pat. No. 4,259,306 (1982); EP No. 55 529 (1982); EP No. 49 386 (1982); JP No. 5 973 423 (1984); JP No. 6 019 1017 (1985); JP No. 6 014 1617 (1985).
The synthetic ferrierites disclosed in the literature are all prepared by hydrothermal treatment of a basic aqueous medium containing a silica and alumina source. In order to obtain the basic pH(&gt;10) necessary for dissolving the silica and alumina sources and for zeolite crystallization one or more inorganic bases as NaOH and(or) alkaline silicates and aluminates are generally used. Quaternary amines or ammonium are often associated to said bases in order to direct the gel crystallization towards ferrierite and to favor an increase of the Si/Al ratio in the zeolite, as illustrated by U.S. Pat. Nos. 4,046,859 and 4,107,195.
This procedure suffers from several disadvantages. The zeolites having a light structure as does ferrierite are generally metastable in their basic formation medium and accordingly are difficult to obtain in a very pure state. They are often accompanied with heavier and more stable phases. This difficulty increases with the amount of product to prepared, prepare, i.e. when passing from the laboratory scale to the industrial scale.
On the other hand, these basic media are not favorable to the crystallization of phases of very high silica content. As a matter of fact, it is well known that the pH increase in the synthesis medium results in a decrease of the Si/Al ratio in the zeolite which crystallizes. In this connection, the solubility in basic media increases with the Si/Al ratio, is, the effect being a reduced yield of the preparations. The metastability of the zeolites requires very highly oversaturated media to obtain their crystallization, thus producing a quick nucleation to the prejudice of the crystal growth, thereby resulting generally in crystals of small size.
Many applications of ferrierite, particularly in acid catalysis, require zeolite in protonated form and entirely freed from its alkaline cations introduced as compensation cations during the synthesis. This can be achieved by processes of repeated and extended ion exchange with NH.sub.4.sup.+ cations followed by roasting to decompose them to H.sup.+ cations. This ion exchange step could be omitted if it were possible to replace the alkali cations by NH.sub.4.sup.+ cations during the synthesis. But this is not possible when the pH is higher than about 10, NH.sub.4.sup.+ being in these conditions converted to NH.sub.3. On the other hand, the synthesis conducted at pH values for which NH.sub.4.sup.+ is stable, are difficult and time-consuming in view of the low solubility of the silica and alumina sources at these low pH values.